Why is it not raining in the Atacama desert

The Atacama Desert , with a length of almost 1600 km, is the driest non-polar desert on the planet , since for just one millimeter of water to rain, it can take between 10, 20 or 40 years. It is located on the South American continent to the north of Chile and covers an area of ​​about 105,000 km ² . It is bounded by the Pacific Ocean to the west and the Andes mountain range to the east.

Its geographical position will be one of those responsible for its extreme aridity, together with the climatic factors of the area, the fact that in the past it constituted the seabed or the high concentrations of salt. All this means that the appearance of this desert transports us almost immediately to fictional films about other planets, such as Mars.

It is rich in various mineral resources, both metallic, such as copper, iron, gold or silver, and non-metallic, such as boron, lithium, sodium nitrate or potassium salts. Another remarkable resource obtained from this region, specifically, from the Atacama salt flat, is bischofita, a magnesium salt used as a caking agent in road construction.

As previously mentioned, together with the Antarctic desert, they make up the so-called hyper-arid zones , which cover 4% of the Earth’s surface and in which it may not rain for years .

For a rain to be measured, it must register a quantity of at least 1 mm of water, which in the Atacama desert only occurs once every 15 or 40 years , having even registered periods of up to 400 years without rainfall in its area. central. However, during the first two months of the year, between January and February, the so-called “highland winter” occurs, during which some occasional rain may occur along with abundant electrical storms. In addition, there are seasons of strong winds that cause tornadoes and blizzards that can easily reach speeds of 100 km / h.

As for its temperatures, these are highly fluctuating, especially at night, reaching down to -25ºC in the Ollagüe area. During the day, temperatures range between 25ºC and 50ºC in the shade . Due to its situation close to the limit of the Tropic of Capricorn, there are no differences between the winter and summer periods. In addition, solar radiation is very high on the scale of the ultraviolet spectrum, which is why the use of sunglasses is essential, as well as sunscreens with UV protection.

The relative humidity of the air is very low in the interior areas (barely 18%), reaching 14% at a depth of one meter below the ground, an extreme value that has not been recorded anywhere else on the planet. However, relative humidity is high in coastal areas, reaching values ​​of up to 98% during winter.

Once we have reached this point, we must find an answer to this great question. The truth is that it is due to climatic and geographical factors.

First of all, we must refer to the so-called Humboldt Current , an ocean current that occurs in South America that originates from the rise of cold and deep waters towards the ocean surface. Due to the latitude at which this continent is located, it would be normal for the climate to be tropical or subtropical, but due to the low temperature of the water the atmosphere and the sea breezes cool down and, therefore, the climate becomes excessively arid. As the water does not evaporate and, consequently, precipitation clouds do not form . This alters the rain regime, which becomes very scarce.

In relation to this, it should also be noted that, due to its situation in the tropic of Capricorn, what happens is that the air that descends from the equator arrives dry and without humidity, having previously dumped abundant rains in this other area.

On the other hand, there is another great person responsible for the aridity of the Atacama desert . It is the Foehn or Föhn effect , responsible for the fact that the winds coming from the east cannot penetrate and leave rain. This phenomenon is typical of mountainous regions. What happens is that when a mass of warm and humid air is forced to rise to overcome the obstacle posed by the mountain, the water vapor cools and condenses, producing precipitation on the mountainside, where the mountains have formed. clouds, in this case, in the Andes Mountains (windward).

What happens next is that, on the opposite slope, the one that faces the desert (leeward), when the clouds pass over the mountain they do not have enough water. The temperature rises and the sky is clear due to the rapid descent down the mountainside of the dry and increasingly warmer air, as a result of the increase in pressure as we descend in height and there is so little humidity. As a result, during the descent of these dry air masses the clouds heat up until they evaporate and disappear. In short, on these lee slopes the Foehn effect results in a very dry and hot wind .

In addition, to the north of the Cordillera the Andean Altiplano forms, an extensive elevated plain that captures moisture from the Pacific Ocean to the south and prevents storms from the Amazon from reaching Chile to the north . With all this, the result we obtain is total aridity and the absence of rainfall in the Atacama desert .

However, despite all the above, there are specific situations in which the rains exceed the normal range of the desert. This occurs only when the El Niño phenomenon alters the rain pattern.

During the spring months in the Tropic of Capricorn, between September and November, when these rains occur, an unusual phenomenon occurs. Seeds, rhizomes, bulbs and tubers that were buried in the soil in a dormant state germinate and flower, covering the surface of the desert with a variety of flowers and filling it with color. It is estimated that there are more than 200 species, most of them endemic to the region, protected underground for years to protect themselves from the extremely arid conditions of the desert, waiting to emerge when it rains.